Method and system for flushing sulphite pulp digesters



June 2, `1970 c. F. ROSNBLAD ETAL 355155631 MEHOD AND SYSTEM FOR FLUSHING SULPHI-TE PULP DIGESTERS y l2 Sheets-.Sheet 1 Filed July 14, 1965 Sneek-Sheet: 2

June 2, 1970 y c. F. ROSENBLAD ETAL METHOD AND SYSTEM FOR FLUSHING SULPHITE PULP DIGESTERS Filed July 14, 196s United States Patent O 3,515,631 METHOD AND SYSTEM FOR FLUSHING SULPHITE PULP DIGESTERS Curt Fredrick Rosenblad, Rosenblad Corporation,

P.O. Box 585, Princeton, NJ. 08540, and Vaino Somer, Kaukas, Finland; Hannu Somer, administrator of said Vano Somer, deceased Filed July 14, 1965, Ser. No. 474,205 Int. Cl. D21c 7/14 U.S. Cl. 162-48 4 Claims ABSTRACT OF THE DISCLOSURE The method and system provide for introducing ushing liquor into a digester through a special tting applied to the bottom of the digester. This is done a short time prior to the blow to lift and break up the lumps of stock in the digester, particularly in the bottom cone thereof, so that the stock will ow out readily when the blow begins. The blowing takes place through the same special fitting. The ushing may continue for some time after the blow starts. Also the ushing initially employs hot liquor and continues with cooled liquor after the blowing commences.

In the chemical pulping process and in the sulphite cooking process particularly, the blowing of the batch digesters clean has been a general problem. This is especially true when such batch digesters are packed too hard with chips. Flushing systems of diiferent kinds have been used to avoid hang-up or clogging in digesters during the blowing period. Some of such prior flushing systems have a pump of ample capacity and head, and spray nozzles, located inside the digester, usually close to the bottom cone of the digester, supplied by such pump. Such systems require expensive pipings and revamping work on the digester, since the supply pipes to the spray nozzles go through the digester shell and the inside brick lining thereof.

The ushing system and method of the present invention do not require any modification of the digesters per se. The revisions of existing apparatus which are necessary in practicing the present invention are simple and economically made. In adapting existing systems for the practice of the present invention, the acid charging pump and piping leading to the digesters can be employed selectively to conduct acid and flushing liquor to the digesters, since acid is added to the digesters only at the beginning of a cook and thus the acid-injecting apparatus is available to introduce flushing liquor into the digesters immediately before and during the blowing operation.

The invention has among its objects the provision of a novel method of ushing the digesters of chemical pulping systems.

A further object of the invention lies in the provision of a novel system for carrying out the above method.

Yet another object of the invention lies in the provision of an apparatus for ushing chemical pulping digesters, such system requiring no changes in the structure of the digesters themselves.

Still a further object of the invention lies in the provision of a flushing apparatus of the type described which employs the existing acid charging mechanism of the pulping system with the addition to the system of a few simple and easily operated elements.

A further object of the invention lies in the provision, in one embodiment of apparatus herein disclosed, of means for automatically controlling the rate of introduction of ushing liquor into the digesters in accordance E icef with the rate of production of blow steam from the blowing operation.

Another object of the invention lies in the provision of a novel method of flushing chemical pulp digesters wherein the temperature of the ushing liquor employed is yvaried so as to preserve the lining of the digesters from undesirable thermal shock.

The above and further objects and novel features of the invention will more fully appear from the following description when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are for the purpose of illustration only, and are not intended as a definition of the limits of the invention.

In the drawings, wherein like reference characters refer to like parts throughout the several views,

FIG. 1 is a schematic view of a rst illustrative system in accordance with the invention for flushing a chemical pulp digester;

FIG. 2 is a view in vertical axial section through the tting at the bottom of the digester through which acid and flushing liquor are injected into the digester;

FIG. 3 is a view similar to FIG. 2 but taken on a sectioning plane which lies at right angles to that of FIG. 2, the view being taken in the direction of the arrows; and

FIG. 4 is a schematic view of a second illustrative system in accordance with the invention for flushing a chemical pulp digester, such system providing for the variation in temperature of the ushing liquor employed in accordance With the rate of production of blow steam from the blowing operation.

In the pulping system schematically illustrated in FIG. 1 there is shown a digester 10 of conventional form and construction. Such digester, which has a frusto-conical lower end '28, is supplied with cooking acid and flushing liquor in a manner to be described through a tting 11 connected to the bottom of the digester. The construction and shape of the fitting 11 are shown in FIGS. 2 and 3, which will be specifically discussed hereinafter. The lower end of the digester 10 communicates with the fitting 11 and a selectively operable blow valve 13 to discharge the contents of the digester at the end of a cook through a blow conduit 12 which discharges such contents into a blow pit or tank 14.

In accordance with conventional practice the blow pit 14 is provided with an exhaust stack 15 through which vapors and noncondensable gases are discharged from the blow pit. The blow pit lis provided with a screen 16 somewhat above the bottom thereof, such screen serving to separate the pulp from the liquor, the latter being discharged in part through a conduit leading to a liquor valve .17 and in part through a further valve 44, to be described. The blow pit is additionally provided with a drainage pipe 18 which is opened at the end of a blow by means of a valve 23. Waste cooking liquor is withdrawn from the bottom of the blow pit 14 below the screen 16 through a pipe 20 in which there are interposed in that order a valve 17 and a pump 19. Liquor is forwarded through pipe 20 by pump 19 to a storage tank 21 from which it is conducted to an evaporator (not shown) by a pump 22 and a conduit 24.

The pulping system shown -is provided with an acid tank 25 from which acid is fed through a selectively operable valve 26 and a pump 27. From the pump 27 the acid travels through a pipe 29 to a header 30 from which acid may be supplied to a plurality of digesters of the system, such digesters being similar to that shown at 10. A further pipe 31 connected to header 30 is connected to the iitting or elbow 1.1 at the bottom of the digester 10 through a valve 32. The construction of the tting 11 is shown in detail in FIGS. 2 and 3.

As there shown the tting 11 is primarily in the form of an inverted T, the body 34 of the fitting having an upper ange 3S with a central opening 36 therethrough by means of which the fitting is connected to the cone 28 at the lower end of the digester 10. The body 34 of fitting 11 has a smaller side branch 37 to which the pipe 31 is connected to feed acid to the interior of the fitting and thence upwardly through the opening 36 therein into the interior of the digester. The body 34 of the fitting .11 has a second, larger branch 39 by means of which the bottom of the digester is connected to the blow conduit 12 through the valve 13. The fitting is further provided with a small branch opening 40 at its bottom, such opening being connected to a drain pipe 41 having a valve 42 interposed therein. The opening 40, pipe 41, and valve 42 provide for the draining of the digester and fitting when desired, as during cleaning of the system. The system is provided with a tank 47 which is supplied with Waste cooking liquor from the bottom of the blow pit 14 through a pipe 46 having a valve 44 and a pump 45 connected therein in that order. The liquor in pipe 47 is available for padding and diluting the stock in the digesters during the subsequent blowing thereof through a conduit (not shown) leading from the tank 47 to the top of the blow pit 1-4.

The above described portion of the system shown in FIG. l operates as follows:

After the digester has been filled with chips, acid is introduced thereinto from tank 2S by starting the pump 27 and opening valve 26. The time for acid charging usually takes -40 minutes, depending on the size of the digester. Total cooking time is in the range between 5--10 hours, depending on the quality of the pulp.

After the cook is finished, the blow valve 13 is opened and the stock is blown from the digester over to the blow pit .14. Blowing time is in the range of 10-20 minutes, depending on digester size. Strong liquor is discharged from the blow pit 14 into the storage tank 21 by opening the valve 17 and starting the pump 19. Simultaneously, as above explained, weak liquor or water is introduced at the top of the blow pit 14 for liquor replacement, by conduit means (not shown) extending from tank 47 to the blow pit, a certain quantity of liquor having been pumped over to tank 47 during a previous blow so as to be available for padding and dilution of the stock during the present blow. This liquor is always recirculating in a closed system; the quantity of such padding and dilution liquor is determined by the size of the digester and the required level of padding liquor to protect the strainer 16 in the blow pit. After this quantity of liquor has been transferred to tank 47, the valve 44 is shut ofi and pump 45 is stopped. After this, the valve 17 is opened and liquor pump 19 is started, thus transferring strong liquor to storage Vtank 21. From tank 21 the liquor is pumped through pipe 24 by liquor pump 22 to an evaporator or plant (not shown) for chemical recovery. When the percentage of solids in the liquor has dropped to a certain pOint, the liquor discharged from the blow pit is switched to the drainage pipe .18 by opening valve 23, closing valve 17, and stopping pump 19.

In accordance with the present invention the waste liquor in the padding and dilution tank 47 is employed to flush the bottom of the digester 10 immediately before and during the blowing of the digester. Thus a pipe 48 is connected to tank 47 and through a valve 50 communicates with the intake port of the acid pump 27 between such port and the acid yalve 26. The acid valve 26 will, of course, have been closed at the start of the cook and remains closed until the start of a next cook. When the digester 10 is ready to blow, the valve 50 is opened and the pump 27 is started. It is to be understood that if there are a plurality of digesters in the system and the time for the acid charging of one digester interferes with the time for blowing another digester, separate pumps similar to pump 27 may be employed for simultaneously injecting acid into the one digester and for flushing the other, the liquor then being introduced through a separate header and pipe line into the elbow or fitting 11 of the digester 10 which is about to be blown.

The blow valve 13 is opened a short time after the iiushing operation has begun. Usually, depending upon the size of the digester and the capacity of the pump 27, the operator has to wait 2-4 minutes after he has opened valve 50 and started pump 27 before he opens valve 13. During such initial period the flushing liquor gives the stock in the bottom cone of the digester a shake-up, that is, it tends to lift it and to break up the lumps therein so that it will flow readily out of the fitting 11 upon the beginning of the blowing operation. Typical flow conditions of the iiushing liquor are around 1600 gallons per minute at 100 pounds per inch gauge pressure. In one embodiment of method in accordance with the invention such rate of ow of the flushing liquor continues until the blow is finished. Not only does such iushing liquor free the contents of the digester for blowing, but it also dilutes the stock to secure a uniform leveling out of the blown digester contents in the blow pit.

The flushing method in accordance with the invention may be conducted on a time schedule, that is, the operator may be instructed to open the blow valve 13 at a predetermined time after he has opened the acid valve 50` and has started the acid pump 27. A still better method of controlling the operation, however, is to employ the increase in pressure in the digester as a means of controlling the blowing operation. Thus, in normal operation, the pres` sure within a digester when it is ready to blow may be down to 25-30 p.s.i.g. Upon the introduction of flushing liquor in the bottom of the digester in the manner de scribed, the level of the liquor in the digester will rise, thus causing compression of the vapor at the top of the digester. Such pressure may rise to 40 p.s.i.g., for example, when it is time to open the blow valve 13. In practising the latter described method, during the flushing operation the operator observes the pressure gauge (not shown) connected to the digester and when it reaches a predetermined reading as, for example, 40 p.s.i.g., he then opens the blow valve 13, opens the valve 17, and starts pump 19. It is to be noted that since the acid injecting and flushing pump 27 may have a 100 p.s.i.g. head and the digester 10' may be designed for, for example, p.s.i.g. working pressure, care must be taken that at no time does the pressure within the digester exceed its working pressure, in this case 70 p.s.1.g.

In FIG. 4 there is shown a system which is particularly etiicient in its recovery of heat from the blow steam from the digester. Such result is accomplished by employing cooled waste liquor as the ushing liquid throughout the predominant portion of the blowing period. At the same time such system protects the digester, which is conventionally made with a metal shell having a brick lining, from thermal shock by initially employing hot waste liquor as the flushing liquid. Additionally, the system of FIG. 4- is shown with automatic controls whereby the introduction of flushing liquor into the digester is automatically started upon the opening of the blow valve 13, and the rate of introduction of flushing liquid is decreased in accordance with the decrease in the formation of blow steam during the blowing operation.

Parts in the system of FIG. 4 which are similar to those of FIG. l are designated by the same reference characters but with added primes. In this system there are provided two storage tanks 54 and 5S which, in effect, replace the single storage tank 47 for the padding and dilution liquor in the system of FIG. l. Tank 54 contains hot strong waste liquor, whereas tank 5S contains cooled strong waste liquor. The tank S4 is supplied with hot strong waste liquor from the bottom of the blow pit 14' through a valve 17', liquor pump 19', a valve 56, and a conduit 46.

Above the valve 56 in conduit 46' there is provided a branch conduit 59 which leads to the intake port of a further liquor pump 60. A valve 63 in conduit 59 permits the conduit to be opened or closed. Pump 60 supplies liquor through a pipe 61 to the intake port of a rst set of channels in a heat exchanger 62, the exit outlet port of said first set of channels being connected to the tank 55 through a conduit 64. Cooling water for the heat exchanger 62 is introduced thereinto through an inlet port 66 from which it flows through a second set of channels which are in heat exchanging relationship with the rst set of channels to an outlet port to which a pipe 67 is connected. The thus heated cooling water may be employed for conventional purposes such as washing and s forth in the plant. As in FIG. l the predominant part of the -waste liquor is discharged from the blow pit 14' by the pump 19 into a conduit 20 which leads to `a storage tank 21.

Liquor from either of the tanks 54 and 55 is selectively discharged to the pipe 48' through conduits 49 and 49' and valves 57 and 65 interposed in such latter conduits, respectively. It will be understood that upon the opening of valve 57 and the closing of shut off valve 65 only hot strong liquor is delivered to pipe 48 from tank 54, and that upon the closing of shut off valve 57 and the opening of valve `65 cooled strong waste liquor is delivered to pipe 48 from the tank 55. The provision of the valve 56 in conduit 46', of the valve 69 in conduit 20', and of the valve 63 in conduit 59 permits all or a part of the waste liquor from the blow pit 14 to be delivered to the tank 54, to the tank 55, or to the tank 21.

The system of FIG. 4 thus rst described operates as follows:

Liquor from blow pit 14 may be pumped by pump 19 directly to tank 54, may be pumped to tank 55 through the heat exchanger 62 and cooled in countercurrent ow by heating process water or other kind of liquor indirectly, and/ or may be pumped to the storage tank 21 upon the required manipulation of valves 17', 56, 63, and 69. The ilushing procedure is similar to what has been described in connection with the system of FIG. 1 except that ilushing liquor is rst taken from the hot liquor tank 54 and later from the cool liquor tank 55. The change from tank 54 to tank 55 as a source of flushing liquid is effected at the same moment when the blow valve 13 is opened. Such change of source of liushng liquid is accomplished by fully closing the formerly open valve 57 and fully opening the formerly closed valve 65. The reason for introducing hot liquor in the initial shakeup period is to protect the inside brick lining in the digester. Thus the brick lining is initially cooled by the hot ushing liquor to a temperature at which it may safely be subjected to the cooler liquor from tank 55 without damage. If the digester should be one having a stainless steel lining, the cooled liquor from tank 55 could be used for the entire flushing period. The introduction of cooled liquor during the blow, in both the above described methods, makes it possible to recover the heat from the blow steam by condensing theV blow steam by the cooled ushing liquor.

As above set forth, in FIG. 4 there is shown a control mechanism by means of which the time of opening and the degree of opening of the llushing liquor valve 50 is automatically controlled. Uncondensed vapors and noncondensable gases, which are formed in proportion to the total formation of blow steam, escape from the blow pit through the stack Thus there is provided a llow responsive means 70 which is connected to the stack 15', as shown. Means 70 is connected by a control line or link 71, which may be electrical, pneumatic, or even mechanical, to a valve controller 72 which is connected to operate the variable shut off valve 50". Such control mechanism is so constructed and arranged that the opening of the blow valve 13 causes the flushing valve 50 to be opened by means 70 through the control link 71 and the controller 72. In the initial portion of the blow, during which the formation of blow steam takes place at a high rate, the valve 50 s opened wide by means 70, 71, and 72, thereby to introduce flushing liquor into the digester at a high rate. As the rate of production of blow steam decreases, the valve 50' is progressively closed by the control means 70, 71, and 72.

Although a limited number of embodiments of the invention have been illustrated in the accompanying drawings and described in the foregoing specification, it is to be especially understood that various changes, such as in the relative dimensions of the parts, materials used, and the like, as well as the suggested manner of use of the apparatus of the invention, may be made therein without departing from the'spirit and scope of the invention, as will now be apparent to those skilled in the art.

What is claimed is:

1.*In chemical pulping of wood chips in accordance with the batch system the method of flushing a digester to prevent hang-up during blowing wherein said digester is equipped with a tting at the bottom thereof for blowing which comprises, introducing hot flushing liquor taken from a previous blow and at a temperature high enough to prevent damage to the lining of the digester into the packed digester through such tting, forcing such liquor under pressure up into said digester through said fitting prior to the commencement of the blow to free the cooked pulp for blowing and blowing the digester after the commencement of introducing of Said flushing liquor.

2. The method as in claim 1 and continuing the introduction of flushing liquor into the digester throughout the duration of the blowing of the digester.

3. The method as in claim 1, comprising commencing the blowing of the digester in response to a predetermined increase in pressure within the digester.

4. In chemical pulping of wood chips in accordance with the batch system the method of flushing a digester to prevent hang-up during blowing wherein said digester is equipped with a fitting at the bottom thereof for blowing which comprises, introducing hot ushing liquor taken from a previous blow and at a temperature high enough to prevent damage to the lining of the digester into the packed digester through such tting, forcing such liquor under pressure up into said digester through said fitting prior to the com-mencement of the blow to free the cooked pulp for blowing and blowing the digester after the commencement of introducing of said llushing liquor, extracting some of the heat from and reducing the temperature of hot flushing liquor by using the same to heat another fluid in a heat exchanger and introducing a liquor so cooled into said digester through said tting to continue the ushing after the blow has commenced.

References Cited UNITED STATES PATENTS OTHER REFERENCES Libby, C. E., Pulp and Paper Science and Technology,

VI, N.Y., McGraw-Hill, 1962. Chapter 10, pp. 254-258, by E. O. Ericsson et al., Sulte Pulping TS 1105 p. 9.

HOWARD R. CAINE, Primary Examiner U.S. Cl. X.R. 162-52 

